The present invention relates generally to electronic equipment utilizing time-domain reflectometry to record measurements, and more particularly, to the modification of an electronic time-domain reflectometry material level sensor for use as either a discrete positioning or continuously variable user input device.
In traditional time-domain reflectometry (TDR) systems, the length of (or location of a fault in) a wire cable can be determined by transmitting an electrical pulse down the length of the cable. Any change in the characteristic impedance of the transmission medium, i.e. the wire cable, will result in a reflection of the electronic pulse. Accordingly, upon reaching the end of the cable (or the location of a fault), the transmitted electronic pulse is reflected, and returns down the length of the cable to the point of origin. Since the velocity of the electronic pulse in the wire cable is known, by measuring the time between the transmission of the electronic pulse and the return of the pulse reflection, the distance to the cable end (or fault location) may be precisely determined.
U.S. Pat. No. 5,610,611, to McEwan describes the construction and operation of a high accuracy electronic material level sensor utilizing TDR principles, and is herein incorporated by reference. The McEwan device, shown in FIG. 1, utilizes a precision quartz crystal time base, an ultrawideband directional sampler, and constant fraction discriminators to obtain highly accurate TDR measurements regardless of material dielectric constants and undesired "ghost" reflections. The accuracy achieved with the McEwan device is on the order of 0.1% or better, allowing the device to be used to determine the level of a material (such as a fluid) stored in a tank. A transmission line or guide wire is partially immersed in the material to be measured, and electronic pulses are propagated along the wire by the associated electronic components. Reflections produced in the wire at the material interface, due to changes in the dielectric constant, are correspondingly received and the transmission time of the pulse to the material interface is recorded. The transmission time is then utilized to determine the level of material within the tank.
The present invention adapts the TDR electronics disclosed in McEwan to replace traditional user inputs to electronic devices. User inputs to electronic devices are typically provided by either rotary or linear potentiometers, non-contact capacitive input systems, or by discrete input buttons which mechanically close a contact between two electrical conductors. These traditional user input devices are subject to reliability problems due to wear, breakage, and exposure to environmental elements such as dust and grime which can impede electrical contact. User input devices utilizing TDR electronics require no moving parts or electrical contacts, and offer significant advantages over traditional input devices.